Now that you mention it, I might stay, but I dunno how much longer...
I just really feel like, since I'm a "normal person" I don't belong with you awesome cosplayers whom I aspire to join ever so much.

"Normal"? Well, there's nothing wrong with being normal (mind you, there's nothing wrong with being a little abnormal either, haha.)! You don't have to be an active cosplayer to be a part of this website. This place *to my knowledge* is really for anyone who is interested in the hobby/community/what have you, whether they are an active participant in it or not. ^^ Nobody (at least I hope nobody) will get mad at you for being on this site and not cosplaying. xD Heck, you can gain a lot of cosplay knowledge from here now, even if your mom won't let you cosplay yet.

Believe me, it took me a long time to get my mom even tolerant of anime itself, much less cosplay. Truth be told, I started my cosplay career by convincing my dad to buy me a cosplay while my mom was away on vacation... (do NOT follow my example, I was 12 at the time! XD)

Regardless, I think the best way to go is to not press the issue (she'll just get frustrated and not want you to cosplay even more), and to work on sewing skills for when she will someday let you, or when you turn 18 and can make your own decisions.